The present disclosure relates to a semiconductor memory device, and more particularly, to a temperature sensor circuit for measuring internal temperature to output digital code or changing a self-refresh period according to the measured internal temperature.
Generally, a temperature sensor circuit utilizes a bandgap reference voltage generator. The bandgap reference voltage generator stably supplies a constant voltage in spite of the variation of temperature or external voltage. The bandgap reference voltage is widely used in a variety of applications requiring a reference voltage, for example, semiconductor memory devices or on-die thermal sensors.
The bandgap reference voltage generator includes a base-emitter voltage (VBE) generating unit and a thermal voltage (VT) generating unit. The base-emitter voltage generating unit is implemented with a diode-connected bipolar transistor and supplies a constant diode voltage. The thermal voltage (VT) generating unit generates a voltage proportional to KT (where K is Boltzmann's constant and T is absolute temperature) using the difference of base-emitter voltages (VBE) of two bipolar transistors. The bandgap reference voltage generator minimizes a temperature coefficient by generating a reference voltage (VREF) signal, where VREF=VBE+KVT.
The bandgap reference voltage generator is named in the sense that the reference voltage is substantially equal to a bandgap voltage of silicon (Si).
FIG. 1 illustrates a circuit diagram of a conventional temperature sensor circuit, FIG. 2 illustrates a graph of an error rate according to the change of temperature in the conventional temperature sensor circuit of FIG. 1, and FIG. 3 illustrates a graph of a base-emitter voltage signal with respect to temperature in the conventional temperature sensor of FIG. 1.
Referring to FIG. 1, the conventional temperature sensor circuit includes a first reference voltage generator 100 and a second reference voltage generator 200. The first reference voltage generator 100 generates a temperature sensing voltage VTEMP that linearly varies with temperature and a first reference voltage signal VREF that maintains a certain level irrespective of a variation of temperature. The second reference voltage generator 200 generates second reference voltage signals VULIMIT and VLLIMIT by using the first reference voltage signal VREF. Here, a level of the second reference voltage signal VULIMIT is higher than that of the second reference voltage signal VLLIMIT.
The temperature sensing voltage VTEMP inversely proportional to temperature is used for temperature sensing. The second reference voltage signals VULIMIT and VLLIMIT are used as a biasing voltage of an analog-to-digital converter (ADC). The ADC converts the temperature sensing voltage VTEMP into a digital code.
In order for accurate temperature measurement, the input range of the ADC is defined by the reference voltages of the bandgap reference voltage generator. An upper limit of the input voltage is defined as VULIMIT and a lower limit of the input voltage is defined as VLLIMIT. The ADC compares a DAC voltage with the temperature sensing voltage VTEMP to determine a digital code. Temperature information is determined according to the digital code. At this point, errors such as Process-Voltage-Temperature (PVT) variation or comparator offset may occur during this procedure.
To reduce these errors, a trimming process is performed at a high temperature. After setting an external temperature to approximately 90° C., the trimming process is performed to make the reference voltage signal VLLIMIT have the same voltage level as the temperature sensing voltage VTEMP. After the trimming process, an error rate decreases at a high temperature, e.g., approximately 90° C.
On the other hand, the error rate increases as temperature decreases. The error rate at a low temperature can be reduced by performing the trimming process once again. However, this involves increasing test time.